(1) Field of the Invention
The present invention relates to an image forming apparatus which forms images on the recording medium by causing the developer to jump thereto and can be applied to a digital copier, the printer in facsimile machines as well as to digital printers, plotters, etc.
(2) Description of the Prior Art
In recent years, as the image forming means for producing a visual image on recording medium such as recording paper etc., in response to an image signal, there have been various disclosures proposed such as Japanese Patent Application Laid-Open Hei 6 No. 227,020 as well as Japanese Patent Application Laid-Open Hei 8 No. 99,433. The image forming apparatuses in these disclosures are ones in which charged particles are placed in an electric field so that they will jump by electric force whilst the potential being applied to the control electrode, having a number of passage holes and located in the jump path, is being varied, to thereby make the developer particles adhere to the recording medium located on the surface of the opposing electrode, thus forming a visual image on the recording medium, directly.
In these apparatuses disclosed in Japanese Patent Application Laid-Open Hei 6 No. 227,020 and Japanese Patent Application Laid-Open Hei 8 No. 99,433, the control electrode is provided with a shield electrode on the side facing the opposing electrode so that it will shield electric effects on the toner support from a plurality of electrode elements and their feeder lines, or has a matrix control configuration for implementing control of jumping of the toner.
An image forming apparatus having the type represented by the above prior art uses a control means for controlling the passage of the charged particles through gates. A single electrode plate having openings at positions corresponding to the gates of the control electrode arranged as the control means on the side facing toner support is provided. However, since the jumping amount of the toner is controlled by the electric field formed between the gate and toner support, the jumping amount of the toner changes as the electric field varies.
In a configuration using a cylindrical sleeve as the toner support and a two-dimensional gate array as the control electrode, the distance between the sleeve and the control electrode varies due to the curvature of the sleeve. At the side areas of the support, the distance from the control electrode is greater than that from the central portion. Accordingly, the electric field at areas to the side is weak so that dots formed in these areas will be low in density causing difficulty in yielding the correct contrast.
As countermeasures against this, some manipulations have been used such as increasing the voltage being applied to the electrode at areas to the side when toner passes through. However, the configuration in which the voltage for controlling the jumping toner is adjusted not only needed an increased number of power sources but, also needed extra high withstanding voltage FETs as the means for switching the voltage. This further necessitated insulation against the high voltage, needing more parts and unavoidably resulting in increase in size and cost of the apparatus.
If the control voltage for controlling the jumping of toner is increased without increasing the withstanding voltage, it is necessary to lower one of the potentials, either the potential to be applied for making the toner jump (to be referred to hereinbelow as the ON potential) or the potential to be applied for prohibiting the toner from jumping (to be referred to hereinbelow as OFF potential). If the OFF potential is set higher, the ON potential must be decreased, resulting in insufficiency of toner transfer and hence producing a blurred image without contrast. On the other hand, if the ON potential is set higher, the OFF potential must be decreased. In this case, the stoppage of the jumping of the toner cannot be correctly achieved, causing background fogginess and hence making it difficult to produce a good image with correct contrast. In the case of a color image forming apparatus, this defect causes insufficiency in reproduction of colors.
To deal with this, an attempt at varying the size of the electrode has been made in the aforementioned prior art. However, the toner supported on the toner support also jumps to areas other than the gates on the control electrode. Most of the toner having reached the control electrode will return to the toner support when the potential of the control electrode is switched. However, there is some toner which stays on the control electrode, and this remaining toner causes the apparent potential of the control electrode to vary, which would cause incorrect jumping of the toner.
Further, when toner has built up on the control electrode after toner jumping has been repeated, i.e., in a state where a large amount of toner is adhering to the control electrode, if the voltage for making the toner jump is applied to the control electrode, the toner particles jumping from the toner support will touch the toner adhering on the control electrode or collide with it. At this moment, if the cohesion between the toner particles is very strong, the toner particles may form an aggregation, clumping and remaining on the control electrode. Similarly, as further toner particles repeatedly jump and adhere to the toner aggregations staying on the control electrode, the aggregations finally build up covering the gates. A further buildup of adhering toner will reach and destroy the toner layer carried on the toner support surface. In this way, the gates become clogged and the state of the toner layer is disordered, causing difficulties in producing a good image or causing insufficiency in reproduction of colors in the case of a color image forming apparatus.